Ductile iron



DUCTILEIRON Arthur- A. Avedisian, Windsor, Cnn., 'assignor to The llaylor & Fenn Co., Hartford, Conn., a corporation of Connecticut No Drawing. Application June22, 1953, Serial No. 363,380

4 Claims. (Cl. 75-123) The present invention relates to so-called ductile iron of the type disclosed in the Millis et al. Patent No. 2,485,760 granted October 25, 1949. More particularly, the invention is directed to a modification of such ductile iron, specifically by the addition of tellurium, to enable the production of thin wall castings therefrom which are free from undesirable surface pinholing.

As described in the aforesaid Millis et al. patent, ductile iron is similar chemically to a grey cast iron except that it has included therein a characterizing proportion of magnesium. As to physical characteristics, ductile iron, in the as-cast condition, contains uncombined carbon in the form of spheroidal particles as distinguished from the flake graphite present in ordinary cast iron, and it is a metal which exhibits exceptionally high strength, ductility and elastic properties compared with ordinary cast iron. Ductile iron has the advantage of castability and is usually cast in accordance with conventional foundry practice in much the same way as ordinary cast iron, its unique physical properties rendering it commercially acceptable to an increasing degree as a replacement for such materials as bronze, malleable iron, steel castings and forgings.

Ductile iron, as presently available, is entirely satisfactory for forming castings in large section sizes, but when it is attempted to cast the metal into shapes having thin sections or walls, a very high percentage of the castings exhibit small voids in the surface referred to as surface pinholing which frequently renders the castings unacceptable for their intended purpose. While surface pinholing apparently does not have an appreciable effect upon the strength of the casting or at least it does not have a material or particularly disadvantageous effect, nonetheless such surface pinholing gives the casting a poor appearance frequently rendering the product unsaleable and resulting in a high percentage of rejects solely from a surface appearance standpoint. The reason for surface pinholing in ductile iron castings of thin section has not been understood and therefore it has been recognized as a serious problem in the industry preventing the wide scale use of ductile iron in a large commercial field.

In accordance with the invention, I have solved the problem of undesirable surface pinholing in ductile iron castings having thin sections by adding a small amount of tellurium to the molten iron prior to casting. The amount of such addition may vary from 1 to 8 grams per hundred pounds of iron depending upon the thickness of the sections being cast.

in the production of ductile iron castings, a molten bath of desired ferrous composition is prepared and to this is added a magnesium containing addition agent, usually in the ladle, just prior to casting. In accordance with the invention, I add the tellurium to the melt just prior to or immediately following the addition of the magnesium addition agent. I preferably employ the element tellurium although compounds of tellurium which will break down in the melt and release tellurium are equally acceptable.

nited States atent 0" 2,749,236 Patented June 5, 1956 ice As previously mentioned, the amount of tellurium which may be added will vary within the range of approximately 1 to 8 grams per hundred pounds of melt depending upon the thickness of the sections to be cast. Since the addition of tellurium is such a small amount percentagewise, it is not practical to useless than about 1 gram of tellurium per hundred pounds of melt and this has been sufficient in all tests to avoid undesirable pinholding. Although greater amounts of tellurium maybe used with certain advantages as explained hereinafter, any-excess over ap proximately '1 to 2 grains is unnecessary to avoid or materially reduce surface pinholing.

While an excess amount of tellurium, i. e. more than approximately 1 to 2 grams per hundred pounds of melt will not defeat the purpose of the invention to avoid surface pinholing, nonetheless such excess may have an undesirable effect upon the appearance of the casting by rendering the surface rough and dirty and giving it what might be described as a sooty appearance. In general, an addition of 8 grams or more of tellurium will have such an undesirable effect in all castings regardless of size and less than that amount may have such effect upon castings of thin section. In general, it is preferred for this reason to limit the amount of tellurium added to approximately 3 grams for castings having a section thickness of less than 1 inch and approximately 6 grams for castings having a section thickness greater than about 1 inch but less than approximately 2 inches.

As a specific example of the practice of the invention and the unexpected and useful results obtained, a group of castings were made in a mold having varying section thicknesses of A to 1 inch using a conventional ductile iron melt. All of the castings exhibited surface pinholing in various degrees and the majority were unsaleable. Then a second group of castings were made with the same size molds using the same ductile iron melt except that immediately after the addition of magnesium in the ladle and prior to pouring, there was added 1 gram of tellurium per hundred pounds of melt. All of the castings were free from surface pinholing and no undesirable elfects were noted. The operation was then repeated with the addition of 2, 3, 4 and 5 grams of tellurium with the same results except that when 4 and 5 grams were added, the castings were rough and dirty. The foregoing operations were then repeated using a mold form having sections of 1 /2 to 2% inches. Ordinary ductile iron produced castings having undesirable pinholes while the castings made with the addition of tellurium in amounts of 1 to 8 grams were free from surface pinholing. When 8 grams of tellurium were added per hundred pounds, however, most of the castings appeared dirty on the cope side.

While it has been found in accordance with the invention that a small amount of tellurium addition of approximately 1 to 2 grams per hundred pounds of melt will suffice to avoid undersirable surface pinholing there is a further advantage of adding tellurium which is more fully realized as the amount of tellurium is increased, namely, that the tellurium will tend to break down the carbides present and will improve shrinkage. Contrary to the teaching in the aforesaid Millis et 211 patent, the addition of tellurium in the amounts referred to herein does not have an undesirable chilling effect or otherwise adversely affect the formation of ductile iron castings of good quality.

Accordingly, there has been devised in accordance with the invention a valuable aid in the production of ductile iron castings, particularly castings of small section. It will be apparent that variations in the method will be suggested to one skilled in the art and all such Variations are intended to be included Within the scope of the following claims.

I claim: V

1. A method of'forming castings of thin section free from surface pinholing from ductile iron comprising forming a ductile iron melt and adding thereto a small quantity of tellurium in the proportion of approximately one to eight grams per hundred pounds of melt, and then pouring the melt into molds.

ZJA method of forming castings free from surface pinholing from ductile iron comprising forming a ductile iron melt and adding thereto a small quantity of tellurium not exceeding six grams per hundred pounds of melt, and then pouring the melt into molds. 3. A method of forming castings free from surface pinholing from ductile iron comprising forming a ductile iron melt and adding thereto a small quantity of tellurium References Cited in the file of this patent UNITED STATES PATENTS 2,316,948 Gagnebin Apr. 20, 1943 2,485,760 MilliS et al Oct. 25, 1949 2,661,279 Wilcox Ct al Dec. 1, i953 

1. A METHOD OF FORMING CASTINGS OF THIN SECTION FREE FROM SURFACE PINHOLING FROM DUCTILE IRON COMPRISING FORMING A DUCTILE IRON MELT AND ADDING THERETO A SMALL QUANTITY OF TELLURIUM IN THE PROPORTION OF APPROXIMATELY ONE TO EIGHT GRAMS PER HUNDRED POUNDS OF MELT, AND THEN POURING THE MELT INTO MOLDS. 